[PATCH v4 5/8] hugetlb: call update_and_free_page without hugetlb_lock

[Mike Kravetz]

With the introduction of remove_hugetlb_page(), there is no need for
update_and_free_page to hold the hugetlb lock.  Change all callers to
drop the lock before calling.

With additional code modifications, this will allow loops which decrease
the huge page pool to drop the hugetlb_lock with each page to reduce
long hold times.

The ugly unlock/lock cycle in free_pool_huge_page will be removed in
a subsequent patch which restructures free_pool_huge_page.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 43 +++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index df2a3d1f632b..be6031a8e2a9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1446,16 +1446,18 @@ static void __free_huge_page(struct page *page)
 
 	if (HPageTemporary(page)) {
 		remove_hugetlb_page(h, page, false);
+		spin_unlock(&hugetlb_lock);
 		update_and_free_page(h, page);
 	} else if (h->surplus_huge_pages_node[nid]) {
 		/* remove the page from active list */
 		remove_hugetlb_page(h, page, true);
+		spin_unlock(&hugetlb_lock);
 		update_and_free_page(h, page);
 	} else {
 		arch_clear_hugepage_flags(page);
 		enqueue_huge_page(h, page);
+		spin_unlock(&hugetlb_lock);
 	}
-	spin_unlock(&hugetlb_lock);
 }
 
 /*
@@ -1736,7 +1738,13 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
 				list_entry(h->hugepage_freelists[node].next,
 					  struct page, lru);
 			remove_hugetlb_page(h, page, acct_surplus);
+			/*
+			 * unlock/lock around update_and_free_page is temporary
+			 * and will be removed with subsequent patch.
+			 */
+			spin_unlock(&hugetlb_lock);
 			update_and_free_page(h, page);
+			spin_lock(&hugetlb_lock);
 			ret = 1;
 			break;
 		}
@@ -1805,8 +1813,9 @@ int dissolve_free_huge_page(struct page *page)
 		}
 		remove_hugetlb_page(h, page, false);
 		h->max_huge_pages--;
+		spin_unlock(&hugetlb_lock);
 		update_and_free_page(h, head);
-		rc = 0;
+		return 0;
 	}
 out:
 	spin_unlock(&hugetlb_lock);
@@ -2291,6 +2300,7 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
 	int nid = page_to_nid(old_page);
 	struct page *new_page;
+	struct page *page_to_free;
 	int ret = 0;
 
 	/*
@@ -2313,16 +2323,16 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 		 * Freed from under us. Drop new_page too.
 		 */
 		remove_hugetlb_page(h, new_page, false);
-		update_and_free_page(h, new_page);
-		goto unlock;
+		page_to_free = new_page;
+		goto unlock_free;
 	} else if (page_count(old_page)) {
 		/*
 		 * Someone has grabbed the page, try to isolate it here.
 		 * Fail with -EBUSY if not possible.
 		 */
 		remove_hugetlb_page(h, new_page, false);
-		update_and_free_page(h, new_page);
 		spin_unlock(&hugetlb_lock);
+		update_and_free_page(h, new_page);
 		if (!isolate_huge_page(old_page, list))
 			ret = -EBUSY;
 		return ret;
@@ -2344,11 +2354,12 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 		 * enqueue_huge_page for new page.  Net result is no change.
 		 */
 		remove_hugetlb_page(h, old_page, false);
-		update_and_free_page(h, old_page);
 		enqueue_huge_page(h, new_page);
+		page_to_free = old_page;
 	}
-unlock:
+unlock_free:
 	spin_unlock(&hugetlb_lock);
+	update_and_free_page(h, page_to_free);
 
 	return ret;
 }
@@ -2671,22 +2682,34 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
 						nodemask_t *nodes_allowed)
 {
 	int i;
+	struct page *page, *next;
+	LIST_HEAD(page_list);
 
 	if (hstate_is_gigantic(h))
 		return;
 
+	/*
+	 * Collect pages to be freed on a list, and free after dropping lock
+	 */
 	for_each_node_mask(i, *nodes_allowed) {
-		struct page *page, *next;
 		struct list_head *freel = &h->hugepage_freelists[i];
 		list_for_each_entry_safe(page, next, freel, lru) {
 			if (count >= h->nr_huge_pages)
-				return;
+				goto out;
 			if (PageHighMem(page))
 				continue;
 			remove_hugetlb_page(h, page, false);
-			update_and_free_page(h, page);
+			list_add(&page->lru, &page_list);
 		}
 	}
+
+out:
+	spin_unlock(&hugetlb_lock);
+	list_for_each_entry_safe(page, next, &page_list, lru) {
+		update_and_free_page(h, page);
+		cond_resched();
+	}
+	spin_lock(&hugetlb_lock);
 }
 #else
 static inline void try_to_free_low(struct hstate *h, unsigned long count,
-- 
2.30.2